In recent years, in a spindle for a machine tool, a demand for high-speed operation increases for improvement of cutting efficiency. In addition, recently, in the spindle, for improvement of production efficiency, a need to correspond to a five-axis machining tool capable of machining a workpiece having a complicated shape without using a plurality of machine tools and without a changeover emerges. In the five-axis machining tool, the spindle or a table turns. Thus, shortening of axial length of the spindle is required due to demands for space saving by shortening of turning radius, power saving by inertia reduction in turning and weight reduction, or the like.
Grease lubrication, oil-air lubrication, oil-mist lubrication or the like is exemplified as a lubricating method which is widely adopted for a rolling bearing for the spindle for the machine tool. Generally, the oil-air lubrication is adopted in the area of high-speed rotation (dmn 800,000 or more). As oil-air lubrication of the related arts, there is known a system of supplying a high-pressure air and fine oil particles from a side surface of a bearing into the bearing by using an oil supplying nozzle piece 101 arranged on the side of a bearing 100 illustrated in FIG. 9A or the oil supplying nozzle piece 101 inserted into a radial through-hole 102a of an outer ring spacer 102 arranged on the side of the bearing 100 illustrated in FIG. 9B.
In this system, an oil supplying component such as the nozzle piece 101 is required additionally, and the number of spindles increases. This leads to an increase in cost of the entire spindle and effort for management. In addition, since the nozzle piece 101 is used, a shape of the outer ring spacer and a structure of a housing become complicated, and effort of designing and machining the spindle increases. Further, since the nozzle piece 101 is provided on the side surface of the bearing in the rotational axial direction, a certain degree of the spacer length is required, and the axial length of the spindle is elongated. Accordingly, size of the machine tool itself increases, weight of the spindle becomes heavier as the axial length increases, and a whirling speed (the whirling speed is a rotational speed calculated from a natural frequency of the spindle, and vibration becomes large when the spindle is rotated in this whirling speed range) of the spindle decreases. In addition, supply of oil particles from the oil supplying nozzle is hindered by an air curtain (the air curtain is a wall of high-speed air flow in the circumferential direction generated by friction between air and inner ring outer diameter surface rotating at high-speed) generated by high-speed rotation. As a result, it is hard to reliably supply lubricating oil into the bearing. As described above, although oil-air lubrication of the related arts is more excellent than the grease lubrication in terms of lubricity under high-speed rotation, as acceleration progresses, correspondence to the acceleration becomes important.
As another oil-air lubrication system, as illustrated in FIG. 10, there is known a system which uses an outer ring oil-supply type bearing 110 in which an oil groove 112 is formed on the outer circumferential surface of the outer ring 111 in the circumferential direction, and a plurality of oil holes 113 directed in a radial direction are formed at the same axial position as the oil groove 112 (for example, see Patent Document 1). In such an outer ring oil-supply type bearing, even when the bearing is used at high-speed rotation, the supply of the oil particles is not hindered by the air curtain. For this reason, it is possible to stably use the spindle even at high-speed rotation.
FIG. 11 is a schematic view of the spindle in each case of the oil-air lubrication using the nozzle piece 101 and the oil-air lubrication with the outer ring oil-supplying specification. The upper half of FIG. 11 is a spindle 120 of the oil-air lubrication with the outer ring oil-supplying specification, and the lower half is a spindle 120A of the oil-air lubrication using the nozzle piece 101. Incidentally, in FIG. 11, reference sign 121 denotes a rotary shaft, and reference sign 122 denotes a rotor of a motor fitted to the rotary shaft 121. In this way, when the oil-air lubrication uses the nozzle piece 101, a spacer having a certain axial length or more is required to supply the lubricating oil from the side surface of the bearing 100. On the other hand, with the outer ring oil-supplying specification, an oil supplying spacer is not required. Thus, the nozzle piece can be reduced in size, and the structure of the spacer can be simplified, and the axial length of the spacer 123 can be shortened compared to the case of the specification using the nozzle piece. Accordingly, in the outer ring oil-supplying specification, it becomes simple to design and machine the spindle and the oil supplying component and manage the components, and the reduction of overall cost can be achieved in designing, manufacturing and managing of the machine tools. Additionally, the axial length can be shortened to reduce the size of the machine tool and improve the whirling speed of the spindle. As described above, the outer ring oil-supply type bearing has many advantages compared with a conventional side surface oil-supply type bearing.